The transmission of multimedia data within channels of limited bandwidth or within channels whose errors are predictable has led to the use of the Shannon separation principle being reconsidered, which recommends that the source coding (compression) and the channel coding (protection) be designed separately.
However, in order to meet the compatibility with existing standards and the deployment of existing architectures where network layers can be present between the source and channel coders, the compression and the protection are kept separated although applied in cooperation. Joint channel source coding ensures that the impact of the errors, practically unavoidable in wireless channels, are taken into consideration by efficiently combining the compression and the protection with regard to the visual display. The conventional algorithms for source bit-rate control proposed in the absence of transmission errors are based on the assumption that the error correction coding (or FEC for Forward Error Correction) tools allow the arrival of packets free from errors at the video decoder. In spite of the particular interest in wireless transmissions or broadcast, these solutions do not take into account either the significant effects of severe distortion introduced by an unavoidable residual bit-error probability in narrow-band transmissions, nor the different sensitivities of the bitstreams.
A first tandem joint coding solution, which follows the principle that video decoders suffer chiefly from packet losses, deals with the question of source bit-rate control in the absence of transmission errors, and the establishment of packet pruning mechanisms (discarded packets). This network-oriented approach does not take into account the possibilities offered by more recent transport protocols such as UDPlite (User Datagram Protocol light) or DCCP (Data Congestion Control Protocol) that enable erroneous payloads to be transferred to the application level, where they are submitted to decoders able to use them.
A second family of tandem joint coding schemes uses FEC tools in order to ensure that the probabilities of bit or packet errors seen by the video decoder are situated below a given threshold. The most efficient schemes choose the channel bit-rates depending on the analysis of the sensitivity of the data streams, the key problem being the evaluation of the sensitivity. The global approaches, based on a discrete cosine transform (or DCT [1]) compression or dedicated for given prediction standards [2] [3], propose a definition of the sensitivity and of its impact on the distortion based on an analytical formula for each frame [1][2] or by an approach of the ‘water-filling’ optimization type. However, these approaches require either an adjustment of the model thanks to numerous tests, hindering easy deployment in practical situations, or do not take the different dependences existing in the bitstream totally into account, which could be due to their generic approach not linked to a standard and/or a given application.
The method according to the invention is notably based on the use of a semi-analytical model predicting the distortion in multimedia data.
In the case of the H.264/AVC standard, the distortion is calculated by estimating the impact of the errors in the different partitions/frames, depending on their respective sensitivity to the errors and on the influence of prediction in order to calculate the distortion of a predicted frame and of a group of images (GOP or Group of Pictures).
When it is used with FEC protection, the method allows the allocation of protection to be specified that minimizes the distortion of a GOP or the distortion of the video sequence (which is a series of several GOPs), by application of the protection bit-rate adapted to the level of sensitivity.
The word “slice” defines a partition known to those skilled in the art.